In recent years considerable attention has been focused on systems for providing wide arrays of video programming to subscribers. A number of commentaries have suggested that future systems will provide 500 channels of video information into each subscriber's home. The trend in development of video distribution, as in many communications industries, is toward all digital transmissions. In particular, a number of systems have recently been proposed for distributing video information in compressed, digital data form.
U.S. Pat. No. 5,247,347 to Litteral et al., the disclosure of which is herein incorporated in its entirety by reference, discloses an enhanced public switched telephone network which also provides a video on demand service using compressed digital data transmissions.
One network has been proposed for the distribution of compressed, digitized broadband information, including video information, to subscribers. That network utilizes optical fiber to connect information sources and subscribers to a broadband ATM switch. The fiber carries a standardized transport stream such as a SONET OC-12 transport stream. The techniques disclosed herein apply to other networks as well, such as ones in which QAM, ADSL, digital cross connect and multiple carrier transport mechanisms are used. An adaptive digital multiplexer/demultiplexer permits unused digital capacity on an optical fiber to be allotted to other video sources. A gateway, called a level 1 gateway, manages the establishment of connections between subscribers and the information sources. A digital entertainment terminal at the subscriber's location provides all necessary user functions for the use of the network.
The transmission of programming, in particular, video programming where wide bandwidth is required, has been known to include a variety of compression techniques. Modern decoders of compressed video information will adapt their operating modes in dependence upon the properties of the incoming signal. It is also often the case that video programming is distributed in multiplexed format which requires that either the decoder or an associated demultiplexer be aware of where in the multiplexed stream particular programs are located.
One of the widely accepted standards for the distribution of video programming is that promulgated by the Organisation Internationale de Normalisation (International Organization for Standardization) under Standard ISO/IEC JTC1/SC29/WG11 entitled "Coding of Moving Pictures and Associated Audio". The particular working group responsible for that standardization effort is also known as the Motion Picture Experts Group (MPEG).
MPEG compression and decompression standards have been evolving. The current advanced version is generally known as MPEG-2. The MPEG-2 standard is hereby incorporated by reference in its entirety. In the language of the MPEG-2 standard, a "program" comprises video source material and audio source material which share a common time base. One or more elementary digital streams representing the source material (e.g. video or audio), as well as other data, are combined into single or multiple streams suitable for storage or transmission. System coding follows the syntactical and semantic rules imposed by the MPEG-2 specification and provides information to enable synchronized decoding without either overflow or underflow of decoder buffers over a wide range of retrieval or receipt conditions.
Under the MPEG-2 standard, an incoming individual video signal and related audio signal are encoded and packetized into video packetized elementary stream (PES) and audio packetized elementary stream, respectively. The video and audio PES from one or more programs are combined into a transport stream for transmission or storage.
The transport stream is designed for use in environments where errors are likely, such as storage or transmission in lossy or noisy media. Transport stream packets are 188 bytes in length. Transport stream packets are generally divided into two sections. The first section constitutes header information and the second constitutes payload. Header information includes, among other information, a synchronization byte, transport scrambling control and a thirteen bit program identification (PID) indication. PID value 0 is reserved as an indication that the packet includes program association table data. PID value 1 is reserved for conditional access such as encryption. PID value 0x1FFF is reserved for administrative (no-op) packets utilized for synchronizing the link. Other program identification numbers are utilized to identify transport packets with the program source from which they originate.
FIG. 1 depicts a transport stream utilizing the MPEG-2 standard. As indicated above, each transport packet in the transport stream carries a program identification number (PID). Packets identified by PID 0 carry program association table information. As illustrated in FIG. 1, a program association table maps a particular program source with PID values assigned to packets associated with that source. Thus, the program association table defines PID identification of program maps for each source of programming in the transport stream. As illustrated, the program map for CBS is found in packets labeled PID 132; the program map for NBC is found in packets labeled PID 87 and so forth.
The program map for CBS illustrated in FIG. 1 as that associated with PID 132 identifies the PID numbers for the packetized elementary streams (PES) for the video and audio channels associated with the particular program. One should note at this time that more than two PID's may be associated with a program. For example, there could be a data channel associated with the program which would include data for closed captioning for the hearing impaired. There could be a number of audio elementary streams for, for example, carrying respective different languages. Thus the programming map provides some flexibility beyond that required for merely associating a single video and audio elementary streams.
Once the program map is known, the program decoder can be set to extract the desired elementary streams (e.g. PID 56 in FIG. 1 and PID 74 shown in FIG. 1) for decoding and presentation. Transport packets of PID=56 represents the video elementary stream associated with CBS' MPEG video system. Within CBS' video elementary stream are video sequence headers which define things like frame rate and resolution. After the video header sequence, the normal video syntax occurs which, in the case of MPEG, includes the normal frames associated with video compression such as I frames, B frames and P frames used in MPEG.
The elementary stream with, for example, PID=74 is the audio stream associated with the video shown in transport packets, such as those, for example, with PID=56 as defined in program map at PID=132.
A typical MPEG-2 transport demultiplexer and decoder is shown in FIG. 2. The data link specific interface 210 converts the incoming data link format (e.g. optical fiber encoded data) to a form (e.g. electrical) for demultiplexing and decoding. Transport stream demultiplexer 220 monitors the PID's of the incoming transport stream packets to identify those selected for decoding and routes those to respective video (230) and audio (240) decoders. Since the video and audio associated with the same program are derived from a common clock, clock control 250 drives both decoders. Note that the transport stream demultiplexer includes functionality, such as that described in connection with the description of the transport stream of FIG. 1, necessary to identify the particular PID's needed to extract information from the transport stream to present the desired program to the user.
In the distribution of video programming, and particularly in the context of video on demand, which can allow subscribers access to a large number of channels of video information, it may be desirable to provide a number of channels, each carrying the same program but with delayed start times. This is illustrated functionally in FIG. 3 in which a program source would presumably play one video repeatedly which could be transmitted over one channel undelayed and over subsequent channels delayed by incremental amounts from the start time of the first channel so that, at any one time, a number of channels carries the same program delayed by differing amounts. The transport stream will additionally include a plurality of broadcast channels. The subscriber will select from those channels available.
Referring again to FIG. 1, when a subscriber tunes, for example, to CBS for the first time, before decoding can take place, a program association table and the program map for the selected channel must be received in the stated order from the transport stream. Thereafter, a packet from the video elementary stream must arrive with header information about the frame rate, and resolution for display of the image and the like before the decoder can be set up to actually decode the payload data associated with the video elementary stream. This process takes a relatively great deal of time even in the high-speed world of digital video transmission. The process must be repeated each time a new channel is selected. As a result, if a subscriber switches channels quickly (channel surfing), in an attempt to glimpse what is on each channel, the set-up time for each channel can be significant and the process of moving from channel to channel likely becomes much slower than a user would find acceptable. One way around this problem is to repeat header information often, but this results in a loss of bandwidth which would reduce the quality of the resultant image.
This delay problem existing in modern video compression systems also exists with respect to encryption. When a particular video program is encrypted, the encryption key is sent only infrequently. When first tuning to an encrypted program source, the subscriber would necessarily need to wait for the encryption key to be downloaded, assuming he is authorized, before decoding of the picture could begin. As a result, the speed at which channels can be changed is further inhibited.
In the distribution of video programming, it is desirable to provide the user with VCR-like functionality, including fast forward, fast reverse and channel pause features. Provision of these features is difficult if one program source is being provided to a number of users in a broadcast-like function, when using a digital transport stream like the ones previously described. An individual subscriber cannot affect the playback, for example, without affecting the playback for all subscribers currently tuned to that program.